When the grade falls away along one side of a house, the foundation stem wall grows taller with it. The wall-framing takeoff now models that as a wedge: give it the tapered run and the extra height at the far end, and it adds half of length times extra height of wall face to the concrete, doubles that for form contact area, and adds verticals at their average extra length.
Checked by hand only: a 240 in run growing 48 in adds 5,760 sq in of face, which at 8 in thick is about 1.1 cu yd of concrete (5,760 x 8 / 46,656), the figure the test asserts. It does not design the retaining wall; past 48 in extra the note says so. No real walkout plan has been run through it yet.